The present invention relates to a thin film deposition, and more particularly to a thin film deposition suitable for the deposition of an insulating film such as an SiO.sub.2 film.
As described in a Japanese patent application specification (Publication No. 56-21836), the so-called bias sputtering method has hitherto been used to flatten the surface of a deposited thin film while depositing the thin film on a substrate. The bias sputtering method, in which sputtering is carried out while applying a bias voltage to a body whose surface is to be coated with a film, has an advantage that the deposition of, for example, an SiO.sub.2 film on the body and the smoothening of the surface of the deposited Si0.sub.2 film are simultaneously achieved. In the bias sputtering method, a film, for example, an Si0.sub.2 film is formed in such a manner that part of the deposited Si0.sub.2 film is etched while depositing Si0.sub.2 on a substrate by sputtering a target made of quartz or the like, and the surface of the deposited Si0.sub.2 film is smoothened on the basis of the fact that the etching rate at the film deposition period depends on the shape of the substrate.
That is, in the bias sputtering method, the etching rate at an angled, sloped or stepped surface portion of a substrate is larger than the etching rate at a flat surface portion of the substrate. Therefore, a projection or eaves at the surface of a deposited film caused by a projection or step at a substrate surface becomes small as the thickness of the deposited film grows, and finally the surface of the deposited film is smoothened.
In order to make the surface of the deposited film sufficiently even by the bias sputtering method, it is required to determine the etching rate at the film deposition period so that a resputtering ratio of 30 to 80% is obtained. Incidentally, the term "resputtering ratio" means a ratio (a-b)/a, where a indicates a deposition rate at a time when a film is deposited without being etched, and b a deposition rate at a time when the film is deposited while being etched. In order to satisfy the above requirement, the RF power supplied to a substrate (namely, a body on which a film is to be deposited) is put in a range from 15 to 40% of the RF power supplied to the target. As a result, the substrate is exposed directly to glow discharge, and thus there arises a problem that the temperature of the substrate is raised not only by the condensation energy of Si0.sub.2 and the collision energy of charged particles, but also by the radiation energy from the glow discharge.
An insulator such as Si0.sub.2 is smaller in sputtering yield and deposition rate, as compared with a metal. Accordingly, in order for the Si0.sub.2 film to have a sufficiently large deposition rate, it is required to make larger the RF power supplied to the target. Thus, the radiation energy from the target also elevates the temperature of the substrate (namely, the substrate temperature).
Further, in the bias sputtering method, sputtering is used for the deposition of film. Accordingly, there arises another problem that the deposition rate is relatively small. It has been earnestly desired to solve these problems.